Heat generating systems that utilize furnaces for purposes of firing fossil fuels have long been employed to produce controlled heat, with the objective of performing useful work therewith. Such work might be in the form of direct work, as with kilns, or might be in the form of indirect work, as with steam generators capable of being utilized in industrial or marine applications or for purposes of driving turbines that are capable of being utilized in order to produce electric power. During the course of such a combustion process, the sulfur in the fuel becomes oxidized to thereby form sodium dioxide (SO2), which becomes entrained in the flue gas that is exhausted from the furnace.
Modern water-tube furnaces suitable for use for steam generation purposes can be of various types including, by way of exemplification and not limitation, central-station steam generators, industrial boilers, fluidized-bed boilers, and marine boilers. In accordance with the mode of operation of circulating fluidized-bed type boilers, a gas is made to flow through a bed of solid particles, said bed of solid particles including a crushed solid fuel, such that the flow of gas therethrough produces forces that tend to separate the solid particles in the bed from one another. Moreover, as the gas flow is increased, a point is reached at which the forces on the solid particles in the bed are just sufficient enough to cause the separation of the solid particles in the bed from one another. When such a point is reached the bed then becomes fluidized, that is, the gas that is between the solid particles permits the solid particles to move freely, thereby giving the bed a liquid-like character. Such a circulating fluid bed is further characterized by the very high solids-mixing rates that are capable of being realized therewith. At higher velocities and with the finer size of the solid particles in the bed, the surface of the fluid bed becomes diffuse asentrainment of the solid particles in the bed increases, such that no longer is the surface of the bed defined. Furthermore, recycling of entrained material to the bed, i.e., from the bed to the combustor and from the combustor to the particle recycle system and then once again back to the bed, is required at high rates in order to thereby maintain the inventory of the solid particles in the bed. Continuing, it is to be noted that the bulk density of the solid particles in the bed will decrease as the height of the combustor increases.
In addition to the crushed solid fuel, such as, by way of exemplification, the finely crushed coal particles to which reference has been had hereinbefore previously, the fluidized bed also typically includes crushed sorbent particles, such as particles of an alkaline earth material, e.g., limestone or dolomite particles, suitable for use for purposes of effecting therewith the capture of sulfur from the SO2 that becomes entrained in the flue gases that are produced during the combustion of the crushed solid fuel, so as to thereby reduce the amount of the airborne sulfur emissions present in the flue gas, which ultimately is directed to the stack. The fluidized bed may also include other solids, such as, for example, unburnt materials, which are entrained in the flue gas that is produced during the combustion of the crushed solid fuel and that is thereafter recycled back to the fluidized bed. During the combustion that occurs in the combustor of the fluidized-bed boiler, both solid fuel particles and sorbent particles are consumed. Thus, the fluidized bed of the fluidized-bed boiler must continuously be replenished with fresh crushed solid fuel particles and fresh sorbent particles. In addition, said fluidized bed may also be replenished with recycled sorbent particles that are extracted from the flue gas that is produced during combustion and that is then recycled to the fluidized bed.
U.S. Pat. No. 6,594,553, which is also assigned to the same assignee to which all of the rights in the present invention are assigned, and which has inventorship that overlaps with the inventorship of the present application, discloses that by pre-treating, i.e., coating, finely crushed limestone particles with an aqueous treatment solution, such as, by way of exemplification and not limitation, an aqueous solution, which includes an inorganic salt, wherein the pre-treatment occurs prior to the introduction of such finely crushed limestone particles into the combustor of the fluidized-bed boiler, the capture of the sulfur from the SO2, which is entrained within the flue gas that is produced during combustion can be enhanced, and, therefore, concomitantly the consumption of sorbent and/or the amount of airborne sulfur emissions can be reduced as a consequence thereof. As disclosed in U.S. Pat. No. 6,594,553, the inorganic salt may be a thermally decomposable sodium compound, such as, by way of exemplification, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium nitrate, or sodium acetate sodium hydroxide. In U.S. Pat. No. 6,594,553, it is also pointed out that it is deemed to be desirable to pre-treat the crushed sorbent particles by injecting the aqueous treatment solution into the pneumatic transport piping by means of which the sorbent particles are conveyed from a suitable storage site to the fluidized-bed boiler. This appears to be attributable to the fact that this facilitates the real time adjustment of the rate and the amount of pre-treatment that takes place as a function of both the feed rate of the sorbent and the amount of SO2 that is entrained in the flue gas. In the industry to which the present invention is applicable the process of coating a sorbent with an aqueous treatment solution is commonly referred to as “wetting”. While U.S. Pat. No. 6,594,553 does not contain any description and/or illustration of an injector that would be suitable for purposes of effecting therewith the injection of such an the aqueous treatment solution, various types of injectors, which are known to those skilled in this art could be used without departing from the essence of the present invention to effect therewith the injection of an aqueous treatment solution, of the types to which reference has been had hereinbefore previously, into the pneumatic transport piping through which the sorbent is conveyed so as to thereby cause the sorbent to be treated.
However, it has been found in the course of experimentation with known injection techniques that, while an atomized aqueous treatment solution can be injected into a pneumatic transport piping through which sorbent is being conveyed in a manner so as to thereby facilitate the proper treatment, i.e., the proper “wetting out” of the sorbent being so conveyed, such an injection can also result in the formation of deposits of the injected materials on to the inner wall surface of the pneumatic transport piping. Such formations of deposits in turn can result in the localized agglomeration of the sorbent particles and ultimately result in the formation of “mortar like” nodules or deposits that become firmly attached to the inner wall of the pneumatic transport piping at a point located downstream of the point at which the injector is located from which the atomized aqueous treatment solution is injected into the pneumatic transport piping. The presence of such mortar like nodules or deposits is operative to produce a pressure drop across the pneumatic transport piping and eventually can result in line plugging.
Accordingly, a need has been found to exist in the prior art for a new and improved injection technique for effecting therewith the injection of an aqueous treatment solution into pneumatic transport piping in which a sorbent is being conveyed in a manner so as to thereby facilitate the proper treatment of the sorbent particles and concomitantly that is also operative to at least reduce the risk that deposits of the injected materials will be formed on the inner wall surface of the pneumatic transport piping when compared to the formation of such deposits when conventional techniques are employed.
The difficulty in achieving the wetting of finely divided dry solid particles with small amounts of liquid so as to provide a uniform surface coating thereon is well known to those skilled in the art. To this end, a number of different formulation studies, for example, have been performed with respect to coal water slurries in order to thereby determine the best way of “wetting out” pulverized coal particles so that they can thereby be dispersed into the water phase. Accomplishing this “wetting out” process is also a challenge in coating pneumatically transported limestone particles with a sodium containing solution while yet maintaining good transport properties.
Powders with good transport properties are known to have a high permeability. That is, to this end the air that is being employed for transport purposes readily percolates through the powder to assist the movement of the powder in the conveying line, e.g., the pneumatic transport piping. Continuing, powders that have a high permeability are known to be readily transportable in conventional pneumatic systems. However, the flow of air in powders that have a low permeability have the tendency to result in the compacting of the material, which in turn results in an increased pressure drop in the conveying line, as well as an increased potential for the conveying line to become plugged. Typically, pulverized limestone and coal, in dry form, exhibit good air permeability and are readily transportable in pneumatic systems. While in the case of fluidized bed type systems, it is known that changes in the transport piping pressure are generally caused by increases or decreases in the amount of the solid powders being transported, and that it is also possible for such pressure changes to be caused by changes in the surface characteristics of the powders that are being conveyed. For example, increasing the moisture content of the powder by less than 1% can result in the permeability of the flowing limestone powder being reduced such as to thereby result in the occurrence of a pressure drop and/or the occurrence of line plugging. In FIG. 9 of the drawings there is illustrated a graph of the effect that increasing the amount of dry limestone that is being conveyed has on the pressure drop of the transport piping.
In the sorbent enhancement of powdered limestone, it is known that the enhancing liquid needs to be well mixed with the pulverized limestone. Furthermore, such mixing generally must occur in less than one second, since this is the period of time that is normally available between the time at point in the flow at which the enhancement liquid is introduced to the pulverized limestone and the time at the point at which the mixture is discharged into the boiler. As will be recognized by those skilled in the art, the rate of mixing is, to a large extent, controlled by the degree of contact between the enhancement liquid and the pulverized limestone, and how quickly the surface of the pulverized limestone particles becomes coated, i.e., is wetted, by the enhancement liquid.
Typically, particles of pulverized limestone are conveyed in a loose agglomerated form, wherein such particles of pulverized limestone are joined together along their crystal faces, edges, and/or corners, thereby resulting in the existence of loose and open structures. Such types of structures possess a large amount of surface area which, depending on the implementation, may need to be wetted-out by a relatively small amount of enhancement liquid. In such a wetting out process, the solid-air interface, which exists in the conveying line prior to the introduction of the enhancing liquid thereinto, is replaced by a solid-liquid interface after the enhancement liquid is introduced into the conveying line. For complete wetting, however, it is necessary to effect the replacement of all of the absorbed air on both the external and internal surfaces of the pulverized limestone particles with the enhancement liquid. To this end, fast, effective wetting-out of the limestone will assist in minimizing any negative changes in the air permeability thereof and in the pneumatic transport properties of the limestone.
Accordingly, a need has been found to also exist in the prior art for a new and improved technique for quickly and effectively effecting therewith the wetting-out of limestone particles in order to thereby minimize any negative changes in the air permeability thereof and in the pneumatic transport properties of the limestone.